Under the Microscope: About 14 years ago we built a house. When it was finished we put in a tarmacadam driveway. The contractor excavated the ground, put in a rough limestone base, packed it down with heavy rollers and, finally, spread and rolled the tarmacadam to produce our smooth black drive.
A few weeks later I noticed two bumps had appeared on the tarmac, down near the road. They got bigger day by day and grew into two little mounds. Then the tarmac cracked on the mounds' peaks, to reveal two mushrooms. Life had not only persisted in such inhospitable circumstances but also flourished with enough strength to buckle tarmacadam. Life is tough stuff. How it arose is probably the biggest unsolved mystery in science.
The fossil record of life on Earth, preserved in rocks, displays a diversity of life forms that increases with time from an apparent beginning about 3.8 billion years ago. Fossilised life of a simple bacterial type is present in some of the oldest rocks on Earth. The conventional scientific wisdom is that chemicals formed and accumulated on the early Earth and eventually and spontaneously organised themselves into the first living cell. All subsequent living things are descended from this first cell, and the history of life on Earth forms a branching tree with a common trunk, all the outcome of that singular origin event almost 4 billion years ago.
There is overwhelming evidence that the branching tree of life developed from a common and a single root. The same biochemical features underlie the stupendous variety of life forms, for example. How the common root arose is not clear, however. The explanation I have given remains a hypothesis supported by some evidence.
Geologists can tell us the likely conditions that existed on the very early Earth. Chemists have simulated these conditions in the laboratory and demonstrated the spontaneous formation of many molecules that are important constituents of biological cells. That is as far as it goes, however. We are still a long way from understanding how such chemicals assembled themselves into that first living and replicating cell.
Two things strike one immediately about this scenario, and both concern probability. Firstly, even the simplest conceivable living cell is still a very complex organisation that must be closely self-regulating. The spontaneous self-assembly of lifeless chemicals to form such organised complexity seems staggeringly improbable. Secondly, even allowing that such an improbable event occurred, the plan of life it stumbled on turned out to be so robust and flexible that it survived the extremely inhospitable conditions of the early Earth to flower into stupendous variety and colonise all of the planet's environmental niches.
Earth was formed about 5 billion years ago. We now know life was present on Earth almost 4 billion years ago. That life began so quickly has suggested to many that its origin under the conditions that existed must have been just about inevitable, as such improbable events should not occur so quickly. This thinking quickly leads to the conclusion that life must also exist on Earth-like planets elsewhere in the universe.
Some scientists have reacted differently to the fact that life appeared so quickly, concluding that it was seeded here from elsewhere - a theory known as panspermia - because by their calculations there simply wasn't enough time for life spontaneously to arise here.
Sir Fred Hoyle (1915-2001), the British astrophysicist, was a strong advocate of panspermia. He likened the likelihood that life spontaneously arose on Earth from lifeless chemicals in the time frame available as equivalent to a tornado ripping through a junkyard and spontaneously assembling a jumbo jet. That life arose so quickly would also imply, if the conventional scientific picture is correct, that there is some deep principle that impels organic systems towards the living cell. Hoyle reckoned that if such a principle exists it should be demonstrable by a competent organic chemist in a half-day's laboratory work. No such principle has been discovered.
The conventionally accepted model of cosmology is the big-bang theory, which holds that the universe began in a massive explosion about 15 billion years ago. Hoyle believed this would allow too little time to overcome the massive improbabilities that lay in the path of the spontaneous generation of life. He rejected the big-bang theory and believed in a steady-state universe: one that always existed, with no beginning. Only the endless time available in a steady-state universe could defeat the massive improbabilities that opposed the spontaneous generation of life.
Hoyle's principal scientific work was to discover how the natural elements were forged in stars. His work diverted him from an atheistic mindset into believing that a purposeful design underpins nature. He did not believe that this designer - God - was all-powerful, however, as He was unable to prevent decay.
It seems to me that we are closer to the beginning than to the end of figuring out how life began. But however it began, let us hope it doesn't end like the mushrooms on my driveway ended. I rewarded their heroic tenacity by pouring poisonous chemicals on them, lest they renew their tarmac-buckling exploits next season.
• William Reville is associate professor of biochemistry and director of microscopy at University College Cork